1. Field of the Invention
The present invention relates to a semiconductor integrated circuit device provided with a driver for supplying an electric current to a circuit that includes an inductance such as a stepping motor, and particularly to a semiconductor integrated circuit device for use in a disk drive apparatus.
2. Description of the Prior Art
As an example of such a semiconductor integrated circuit device, a description will be given on a semiconductor integrated circuit device for processing a head movement drive signal as well as to-be-recorded and read-out signals in a disk drive apparatus. This semiconductor integrated circuit device is provided with a stepper block for supplying an electric current to a stepping motor to move a magnetic head, a logic section for reading and writing data from and to a magnetic disk through the magnetic head, a signal path for connecting the logic section to a signal input pad, and a protection circuit for protecting the inside circuitry against a surge voltage that may be applied to the pad from outside. The protection circuit is usually composed of two diodes connected in series between the power source voltage and ground, with the node between them connected to the signal path (to the pad).
The stepper block has an output circuit that is composed of, for example, a first and a second transistor, both being of the NPN type. The first transistor has its collector connected to the power source voltage, and the second transistor has its emitter connected to ground. The emitter of the first transistor and the collector of the second transistor are connected via an output pad to the coil of the motor. Both transistors receive a drive voltage at their base. If the above-mentioned protection circuit is provided near this output circuit, the logic section may malfunction for the following reason. The current supplied to the stepping motor causes the inductance of the stepping motor to produce back electromotive force, which then makes the collector voltage of the second transistor lower than the ground level. This turns on the protection diode that is formed nearby, and thus causes the logic section to erroneously recognize receipt of, for example, a low-level signal and output a false control signal.
Next, a description will be given, with reference to FIG. 6, as to why the protection diode is turned on when the collector voltage of the second transistor becomes lower than the ground level. As shown in FIG. 6, on a P-type semiconductor substrate 69, P-type, N-type, insulating, and other layers are formed by impurity diffusion, epitaxial growth, or a similar method in the following manner. For a protection diode 54, an N.sup.+ layer 70 is provided on the substrate 69, and, on top of this N.sup.+ layer 70, an N layer 73 is provided. In addition, another N layer 71 is provided through the N layer 73. The two N layers 71 and 73 are of different concentration.
The N layer 71 is connected via an aluminum deposit 72 to a pad 58. Part of the N layer 73 is put in contact with a P layer 75 so that a PN junction of the diode is formed. The P layer 75 is connected via an aluminum deposit 81 to ground (GND).
Part of the N layer 73 is separated by an insulator 76. The P layer 75 is also provided with an insulator 77. In addition, insulating layers 78 to 80 are provided on the top surface for the separation of the aluminum deposits 72 and 81.
On the other hand, as an NPN-type transistor 131a for the output circuit of the stepper block, an N.sup.+ layer 85 is provided, as an embedding, on the P-type substrate 69. On top of this N.sup.+ layer 85, an N.sup.- layer 87 is provided. Another N.sup.+ layer 86 is provided through the N.sup.- layer 87 so as to contact the N.sup.+ layer 85, and, Ad on top of the N.sup.+ layer 86, an N layer 82 is provided.
The N layer 82 is connected via an aluminum deposit 88 to a pad 125, which is intended to be connected to the stepping motor. The symbol L represents the inductance component included in the stepping motor. The N layer 82 serves as the collector (C) terminal of the transistor 131a.
An insulator 95 is provided in the N.sup.- layer 87 to separate the N.sup.+ layer 86 and the N layer 82 from a P layer 90. The P layer 90 includes, as part of itself, a P.sup.+ layer 89. An N.sup.+ layer 91 is provided in the P layer 90 so as not to contact the P.sup.+ layer 89. The P.sup.+ layer 89 and the P layer 90 serve as the base (B) of the transistor 131a, and are connected via a deposit 92 to a control circuit 33a.
The N.sup.+ layer 91 serves as the emitter (E) of the transistor 131a, and is connected via an aluminum deposit 93 to ground (GND). Insulators 94 to 96 and 76 are oxide films provided for the separation of circuit elements, and P layers 101, 102, and 75 are diffusion layers provided also for the separation of circuit elements. Insulating layers 78 to 80 and 97 to 100 are provided for the separation of deposits 88, 92, and 93.
Now, suppose that the inductance (L) of the stepping motor 2 has produced back electromotive force, thereby making the voltage at the collector (C) of the transistor 131a lower than the ground (GND) level.
In particular, when the voltage at the collector (C) of the transistor 131a is lower than the ground (GND) level by more than about 0.7 V (the forward voltage V.sub.F), the parasitic transistor 60 (of the NPN type) hidden in the semiconductor integrated circuit device produces leak currents I.sub.1 and I.sub.2, which are approximately equal to each other, and these currents turn on the protection diode 54. As a result, as described previously, the logic section, by erroneously recognizing receipt of a low-level signal, is led to malfunction.